Integrated circuits and other microscopic or nano-scale devices are often processed using charged particle beams for imaging or altering a work piece. For example, focused or shaped ion beams can be directed toward a work piece to micromachine it or to deposit material by beam-induced deposition. Electron beams can similarly be used to etch, deposit, or form images of the work piece using scanning or transmission electron microscopy.
In one application, an ion beam is used to mill a work piece to expose buried features of a work piece to view or modify the buried feature. For example, layers of an integrated circuit may be milled away to expose a buried conductor, which may then be severed or connected to another conductor by ion beam deposition of a new conductive pathway. Ion beams are also used to expose a cross section of a work piece so that the cross section can be viewed using charged particle beam microscopy. A cross section exposed by an ion beam can then be observed using scanning ion microscopy or electron microscopy. For example, U.S. Pat. Pub. No. 20040158409 of Teshima et al. for a “Defect Analyzer” describes methods of cutting cross sections using a focused ion beam. In one technique, referred to as “slice and view,” a cross section is exposed by focused ion beam machining and an image is formed of the cross section. Additional material is then removed from the cross section wall to expose a new wall face offset slightly from the first wall face, and an additional image is formed. By sequentially removing a small amount of material from the cross section wall and forming a series of images, information about a three dimensional structure is obtained. Ion beam machining can be facilitated by using an etch-enhancing gas, such as chlorine, fluorine, or iodine, as described in U.S. Pat. No. 5,188,705 for “Method of Semiconductor Manufacture” to Swanson et al.
Ions beams are also used to extract samples from a substrate for viewing in a transmission electron microscope, as described, for example, in U.S. Pat. Pub. 2004/0251412 of Tappel for “Method and Apparatus for Manipulating a Microscopic Sample.” A sample is freed from the substrate by milling around the sample and undercutting the sample. The sample is typically attached to a probe and moved to a TEM sample holder. The sample also may be thinned or shaped by the ion beam.
The impact of the ions or electrons in a charged particle beam can damage sensitive work pieces. It is a common practice, therefore, to apply a protective layer over the surface of the work piece before performing charged particle beam processing.
Also, non-conductive work pieces tend to accumulate electric charges during charged particle beam processing, both from the charges delivered by the beam and from the ejection of secondary charges caused by the impact of the beam. When a work piece becomes charged, it deflects the charged particles in the primary beam, thereby reducing resolution. Work piece charging can also reduce the number of secondary particles emitted upon impact of the charged particle beam. Because the secondary particles are often used to form images of the work piece, the reduction in the emission of secondary charges degrades the ability of the system to form an image of the work piece. One method of reducing sample charging, described in U.S. Pat. No. 4,639,301 to Doherty for “Ion Beam Processing,” involves the use of an electron flood gun to neutralize accumulated positive charges. Another method of preventing the accumulation of charge is by depositing a conductive layer onto the surface of the work piece to provide a path through which the electrical charge can be discharged.
One method of depositing a protective coating, whether conductive or non-conductive, is by charged-particle-beam-induced deposition. In charged-particle-beam-induced deposition, a charged particle beam is scanned over the work piece surface while a precursor gas is introduced in the vicinity of the beam impact area. The precursor gas molecules are chemically decomposed in the presence of the beam to leave a coating on the surface. Volatile decomposition products float away from the surface and are removed by the system vacuum pump. For example, in ion-beam-assisted gas deposition of a conductive coating, a charged particle beam of between about 5 kV and 30 kV is scanned over the surface while a precursor gas, such as a metaloorganic gas, for example, tungsten hexacarbonyl, is directed to the substrate in the vicinity of the beam landing area. Either an ion beam or an electron beam can be used to decompose the precursor gas to deposit a coating, but electron beams cause less surface damage because electrons have significantly less mass than ions. Although electron beams cause less surface damage than ion beams, the electron beams can still damage some sensitive materials, such as the photoresist or low-k or ultra low-k dielectric layers that are used in integrated circuit fabrication. The beam-induced damage can cause changes in the dimensions being measured on the work piece and changes in the profile being observed.
Another method of covering a surface with a layer of a material is sputtering. Sputtering, also known as physical vapor deposition, is a physical, as opposed to chemical, process in which molecules or atoms are knocked from a material source by momentum transfer and are then deposited onto a target surface. Sputter coating systems are available commercially and are used in integrated circuit fabrication. Such systems are typically designed to deposit a metal layer over an entire wafer. The metal layer is then patterned using a photolithography process to form a conductive pattern to connect elements of the circuit. Such systems typically use a plasma of ionized argon gas, the argon ions colliding with a target to knock material from the target onto the work piece. Such systems typically use electric fields to provide energy to ionize the gas, and may use magnetic fields to trap electrons to facilitate ionization. The use of these production systems to sputter coat samples for quality control or defect analysis may be prohibitively expensive. Other systems, such as the Gatan Ion Sputter Coater can provide a sputter coating on small samples, but will not accommodate a typical semiconductor wafer, and so the wafer must be broken. In either case, coating requires moving the work piece between the sputter system and the charged particle beam system, which can entail multiple evacuations of vacuum chambers, potential contamination as the work piece is moved between system, and additional time and manpower.